Referring to FIG. 1, it shows a conventional RGBW display 10. The conventional RGBW display 10 comprises a plurality of RGBW pixels 11. Each RGBW pixel 11 comprises a green sub-pixel 111, a red sub-pixel 112, a blue sub-pixel 113 and a white sub-pixel 114 arranged in a 2×2 matrix. The conventional RGBW display 10 has the merit of enhanced color space and improved brightness and contrast, compared with traditional RGB display. U.S. Pat. No. 4,892,391, U.S. Pat. No. 5,757,452, U.S. Pat. Nos. 7,286,136, 7,742,205, and U.S. Pat. No. 7,583,279 teach RGBW displays. However, the conventional RGBW display 10 has the deficiency of dark yellow. U.S. Pat. No. 4,800,375, U.S. Pat. No. 7,864,271, and U.S. Pat. No. 8,749,727 teach RGBY displays which claim to have better yellow color. However, RGBY display lacks the merits of RGBW displays. U.S. Pat. No. 7,248,314, U.S. Pat. No. 7,995,019, U.S. Pat. No. 8,248,440, U.S. Pat. No. 8,441,601 and U.S. Pat. No. 8,558,857 teach displays with five color sub-pixels. However five color displays have the deficiency of high cost and low brightness and contrast. U.S. Pat. No. 8,384,653 teaches a method to relief the dark yellow problem by classifying the image signal and then adjusts the backlight luminance according to the classification.